//Given a linked list, swap every two adjacent nodes and return its head. You 
//must solve the problem without modifying the values in the list's nodes (i.e., 
//only nodes themselves may be changed.) 
//
// 
// Example 1: 
//
// 
//Input: head = [1,2,3,4]
//Output: [2,1,4,3]
// 
//
// Example 2: 
//
// 
//Input: head = []
//Output: []
// 
//
// Example 3: 
//
// 
//Input: head = [1]
//Output: [1]
// 
//
// 
// Constraints: 
//
// 
// The number of nodes in the list is in the range [0, 100]. 
// 0 <= Node.val <= 100 
// 
// Related Topics Linked List Recursion 👍 5282 👎 260

  
package leetcode.editor.en;

import listnode.ListNode;

public class _24_SwapNodesInPairs{
    public static void main(String[] args) {
         Solution solution = new _24_SwapNodesInPairs().new Solution();
        ListNode node1 = new ListNode(1);
        ListNode node2 = new ListNode(2);
        ListNode node3 = new ListNode(3);
        ListNode node4 = new ListNode(4);
        node1.next = node2;
        node2.next = node3;
        node3.next = node4;
        ListNode node = solution.swapPairs(node1);
    }
    //leetcode submit region begin(Prohibit modification and deletion)
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode swapPairs(ListNode head) {
        if (head==null||head.next==null) {
            return head;
        }
        ListNode result = head.next;
        ListNode pre = null;
        while (head != null && head.next != null) {
            ListNode nextNext = head.next.next;
            ListNode next = head.next;
            next.next = head;
            head.next = nextNext;
            if (pre!=null){
                pre.next = next;
            }
            pre = head;
            head = nextNext;
        }
        return result;
    }
}
//leetcode submit region end(Prohibit modification and deletion)

}